1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor to be used in an electrophotographic system image forming apparatus, in particular, an image forming apparatus having a high resolution of 1200 dpi, and a high-resolution image forming apparatus including the electrophotographic photoreceptor.
2. Description of the Related Art
Electrophotographic system image forming apparatuses (also referred to as “electrophotographic devices”) that form images using electrophotographic technologies are used as copying machines, printers, facsimile machines and the like to a great extent.
Electrophotographic photoreceptors (hereinafter, also referred to as “photoreceptor”) that are used in electrophotographic processes have a structure including a photosensitive layer containing a photoconductive material stacked on a conductive support.
Photoreceptors including a photosensitive layer containing an inorganic photoconductive material as a main component (also referred to as “inorganic photoreceptors”) have been used widely, but they have a shortcoming in any of heat resistance, storage stability, toxicity to the human body and environment, sensitivity, durability, occurrence of image defects, productivity, production costs and the like. That is, no conventional inorganic photoreceptors have been satisfactory in every way.
On the other hand, research and development have been promoted for photoreceptors including a photosensitive layer containing an organic photoconductive material as a main component (also referred to as “organic photoreceptors”) and the organic photoreceptors have been becoming the mainstream of photoreceptors nowadays.
The organic photoreceptors have some problems in sensitivity, durability and environmental stability. However, they have more advantages compared with the inorganic photoreceptors in terms of toxicity, production costs, degree of freedom of material design and the like. In the organic photoreceptors, for example, their photosensitive layers can be formed by an easy and inexpensive method represented by a dip coating method.
For the organic photoreceptors, there have been proposed: a structure in which a monolayered photosensitive layer obtained by dispersing a charge generation material and a charge transport material (also referred to as “charge transfer material”) in a binder resin (also referred to as “binding resin” or “binding agent resin”) is stacked on a conductive support; and a structure in which a multilayered photosensitive layer composed of a charge generation layer obtained by dispersing a charge generation material in a binder resin and a charge transport layer obtained by dispersing a charge transport material in a binder resin formed in this order or a reversely multilayered photosensitive layer composed of such a charge generation layer and such a charge transport layer formed in a reverse order is stacked on a conductive support. Out of these photoreceptors, function separation type photoreceptors having a multilayered photosensitive layer and a reversely multilayered photosensitive layer have been widely in a practical use, because they are excellent in electrophotographic characteristics and durability, and allow design variation for characteristics of the photoreceptors as having a higher degree of freedom of material selection.
In recent years, as digitization of image information has been promoted, semiconductor lasers and LED arrays have been used instead of conventional white lights as a recording light source (also referred to as “photosensitizing light source”) for exposing a photosensitive layer of a photoreceptor. Currently, as the recording light source, near-infrared laser light sources having a wavelength of 780 nm and red light sources having a wavelength of 650 nm are frequently used.
When digitized image information such as characters is directly used as computer output, the image information is recorded on a photoreceptor according to the computer output information that is converted into a light signal. When image information of a document is input, on the other hand, the image information of the document is read as a light signal, the light signal is converted into a digital electrical signal, and then the digital electrical signal is converted into a light signal again to record the image information on a photoreceptor according to the light signal.
In either case, a part of the photosensitive layer irradiated with a fine spot of light applied from an optical recording head, a recording optical system or the like is developed with a toner to record the image information on the photosensitive layer.
An image is expressed by a group or an array of fine dots developed with a toner. Such dots are called pixels. The optical recording head, the recording optical system and the like have therefore been developed so, as to give higher resolution in order to form a spot that is as fine as possible to allow image information to be recorded with a higher density.
Regarding the optical system for recording image information on a photosensitive layer, there have been developed a variable spot laser recording system, a multi-laser beam recording system and an ultraprecise and ultrahigh-speed polygon mirror (see, for example, p. 117-120 of “New Method of Joining a Polygon Mirror Using a Shrink Fitter” written by Isami NITTA, Kimio KOMATA, Daisuke KONNO, collected papers of Japan Hardcopy '96, 1996. As a result, an optical system for recording image information on a photosensitive layer with a recording density of 1200 dpi or more (dots per inch) has been developed presently.
Even with the development of such an optical system for recording image information on a photosensitive layer with a higher density, it is not necessarily easy to record image information on a photosensitive layer as an electrostatic latent image with good reproducibility. This is attributed to the fact that the light intensity distribution of laser beams is a Gaussian distribution, having a peak in a central part and a spread in an outer part. That is, it was difficult to aim at higher image quality in conventional high-sensitive photoreceptors, because they are exposed also to light having a spread in an outer part to be developed to cause spread of dots.
As high-sensitive photoreceptors, for example, the specifications of Japanese Patent No. 1950255 and Japanese Patent No. 2128593 propose a photoreceptor using a Y-type crystal oxotitanylphthalocyanine as a charge generation material, and Japanese Unexamined Patent Publication No. HEI 10(1998)-237347 proposes a photoreceptor using a novel crystal type oxotitanylphthalocyanine as a charge generation material.
In addition, as photoreceptors using two or more kinds of phthalocyanines as a charge generation material for the purpose of attaining higher sensitivity around 780 nm, which is an emission wavelength of semiconductor lasers, for example, Japanese Patent No. 2780295 proposes a photoreceptor using a mixed crystal of an oxotitanylphthalocyanine and a metal-free phthalocyanine, and Japanese Patent No. 2754739 proposes a photoreceptor using a composition of an oxotitanylphthalocyanine and a metal-free phthalocyanine.
However, these high-sensitive photoreceptors have high sensitivity to weak exposure, too. Accordingly, they cannot achieve higher resolution for the above-described reason.
Further, as photoreceptors using a mixture of two kinds of phthalocyanines as a charge generation material for the purpose of attaining higher resolution, for example, Japanese Unexamined Patent Publication No. HEI 5(1993)-134437 proposes a mixture of two kinds of specific crystal type oxotitanylphthalocyanines, and Japanese Patent No. 3005052 and Japanese Unexamined Patent Publication No. 2002-131954 propose a mixture of a specific crystal type oxotitanylphthalocyanine and a metal-free phthalocyanine. Such photoreceptors are less prone to light decay by weak exposure and high-sensitive to strong exposure to fall into complete potential decay. That is, they have high sensitivity, responding linearly to exposure energy.
While approach to higher-quality images has been developed from a view point of photoreceptors as described above, image forming apparatuses such as copying machines, printers and the like need to provide stable output of beautiful images under various environments, and photoreceptors to be mounted in such image forming apparatuses are required to have appropriate stability, accordingly.
In the meantime, there is a problem in the attempt to attain higher-quality images: fine black dots are generated in an unexposed region.
The fine black dots are an image defect that are generated in a photoreceptor in which a photosensitive layer is directly stacked on a conductive support, because in such a photoreceptor, carrier injection is likely to occur from a side of the conductive support, and surface charges of the photoreceptor, when charged, disappear or decrease microscopically even in a dark place.
In a high-sensitive photoreceptor, in addition, its charge generation material itself has high sensitivity to easily generate carriers even in a dark place due to thermal excitation, also leading to generation of the fine black dots. The image defect of such fine black dots is more significant under an environment of higher temperature and higher humidity.
To prevent the image defect of such fine black dots, to cover defects on the surface of the conductive support, to improve chargeability, to enhance adhesion of the photosensitive layer, and to improve coatability, an undercoat layer is disposed between the conductive support and the photosensitive layer.
Conventionally, various resin materials and resin materials containing, for example, inorganic compound particles such as titanium oxide powders have been considered as the undercoat layer.
Known examples of the resin materials include polyethylene resins, polypropylene resins, polystyrene resins, acrylic resins, vinyl chloride resins, vinyl acetate resins, polyurethane resins, epoxy resins, polyester resins, melamine resins, silicon resins, polyvinyl butyral resins, polyamide resins; copolymer resins including two or more types of these repeat units; polyvinyl alcohol and ethylcellulose.
Out of these resin materials, it is disclosed that the polyamide resins are particularly preferable (see, for example, Japanese Unexamined Patent Publication No. SHO 48(1973)-47344).
In a photoreceptor having a monolayered undercoat layer of a resin such as a polyamide resin, however, residual potential accumulates greatly to reduce sensitivity and cause image fogging, while generation of the fine black dots is inhibited. Such tendency is particularly significant under a low-humidity environment.
Therefore, in order to prevent generation of image defects attributed to the conductive support and improve a variation of the residual potential according to environmental fluctuation, Japanese Unexamined Patent Publication No. SHO 56(1981)-52757 proposes to contain surface-untreated titanium oxide powders in the undercoat layer, Japanese Unexamined Patent Publication No. SHO 59(1984)-93453 proposes to contain titanium oxide microparticles coated with alumina or the like in the undercoat layer to improve dispersibility of titanium oxide powders, Japanese Unexamined Patent Publication No. HEI 4(1992)-172362 proposes to contain metal oxide particles surface-treated with a titanate coupling agent in the undercoat layer, and Japanese Unexamined Patent Publication No. HEI 4(1992)-229872 proposes to contain metal oxide particles surface-treated with a silane compound in the undercoat layer.
As described above, many photoreceptors enabled for high resolution and many photoreceptors aiming at improvement in environmental stability have been proposed. However, they are still insufficient for stable maintenance of high resolution under various environments.
Examples of a means for attaining higher resolution include a method in which a low-sensitive photoreceptor is used so that the sensitivity to light in a region around a region to be exposed is lower and the photoreceptor is exposed only to strong light in the center to form dots precisely. However, this method is compatible with low-speed printers, but incompatible with recent high-speed printers. That is, the problem is that the photoreceptor is low-sensitive to need a high-power semiconductor laser, have high residual potential, and significantly rise in residual potential when used repeatedly, leading to low image density.
In addition, there is another problem: in a high-sensitive photoreceptor using a phthalocyanine as a charge generation material, the high sensitivity leads to more carrier generation due to thermal excitation to cause generation of fine black dots in the unexposed region, which is significant under a high-temperature and high-humidity environment. This problem can be avoided to some extent by providing an undercoat layer, but it is not sufficient because the problem has become relatively significant and noticeable as the resolution is improved, though it was not so serious in conventional low-resolution machines. In addition, depending on the kind of the undercoat layer, introduction of an undercoat layer may lead to reduction in the sensitivity under a low-humidity environment.
Thus, as the resolution of image forming apparatuses is improved, things that were not considered a problem before can now cause a defect, which cannot be handled by prior art technologies.